Method and material for coating and cleaning metal



Patented Feb. 27, 1934 UNITED STAT s PATENT OFFICE DIE'I'HOD AND' MATERIAL FOR. COATING AND CLEANING METAL No Drawing. Application November 17, 1980,

Serial No. 496,328. Renewed July 21, 1933 7 Claims.

The invention relates to a method and material for coating surfaces of metal, such as iron, steel, zinc or magnesium, or their alloys, with substantially insoluble phosphates.

I; An object of the invention is to provide a method, and material by which a coating of the kind described can be formed quickly and with certainty. Another object of the invention is to provide a method and material by which a coatio ing of the kind described can be produced at temperatures less than boiling. The object of the invention is chiefly to prepare metal surfaces for receiving paint or enamel, although the coating produced by the method may be so formed In or treated as to become a satisfactory finish coat in itself under some circumstances. Other objects and details of the invention will appear as the description proceeds.

While the invention allows considerable variation in the composition employed, the temperature at which it is used, and the manner of application, there will be first described the exact composition at present preferred for use as a bath, and then modifications which are desir- 25 able or possible under various circumstances, and the approximate effects of such modifications, will be indicated sufiiciently to enable those skilled in the art to employ the invention in a practical manner under varying conditions and so for various purposes.

A suitable solution may be formed by adding to 4 liters of water, 140 grams of zinc dihydrogenphosphate and 28 grams of copper nitrate. Metal to be coated may be clipped in such a solution. When first made up, this solution should be allowed to stand for some time in order that hydrolyzation may take place and the solution may be brought to an approximate equilibrium.

This will occur in about 2 hours if the solution is kept around 70 F. At higher temperatures, the results will be accomplished in less time, but

for reasons which will be statedlater it is preferable to keep the solution at about the 'tem perature at which it is to be used.

After the solution has been allowed to stand until it has reached approximate equilibrium, the work may be dipped into the solution and allowed to remain there until a coating is formed. Since no appreciable amount of hydrogen or other gas '50 is given ofi during the coating reaction, the time ,of reaction cannot be ascertained by the giving off of bubbles. With the solution described at 70 F., usual clean iron work is thoroughly coated in 2 minutes. The exact time of coating of course varies with the kind of metal or alloy being coated and also with the character of the surface, an extremely hard smooth surface requiring longer than the average surface.

A solution formed with the same ingredients in the same proportions may be used at a lower Q temperature if desired or at any more elevated temperature up to boiling. The time of reaction of course varies with the temperature. In the boiling solution a thorough coating is formed in ten or fifteen seconds on the average article. .5

The temperature at which the solution is kept can be determined in view of the desirability of speed of reaction on the one hand and the trouble of maintaining a raised temperature on the other hand. The solution can be used readily at ordinary temperatures, and is advantageous in that respect where heating means has not been installed. It should be noted further that, where the temperature is not to be raised to any considerable degree, the walls of the tank for holding the solution may be readily lined with tar or other material which will effectively prevent any reaction of the solution with the walls of the tank.

In a solution such as described, it kept at about F., an acidity test made with the use of methyl orange and phenolphthalein indicator gives approximately the following results. Ten cc. of the solution will require about 1 cc. tenth nor-= mal solution of sodium hydroxide to affect the methyl orange indicator and about 31 cc. tenth normal solution of sodium hydroxide to affect the phenolphthalein indicator. For convenience this will be referred to as a ratio of 31 total acidity to 1 free acidity, and a solution of this total acidity will be referred to as a 31 point solution.

When the temperature is raised, greater hydrolyzation results so that the ratio of free acid to total acidity is higher. It happens however, that at the higher temperature a proper reaction requires a higher free acid content in proportion to total acidity, and the prcmortions and materials given result in automatic adjustment of the proportion of free acid to total acidity in response to elevation of the temperature.

At any given temperature and strength-of solution, total acidity to free acid must be kept within fairly narrow limits in order to produce a satisfactory coating. With the solution otherwise as described, it has been found that a reduction of total acidity in proportion to free acid to 25 to -1 or less will still produce a coating but not as satisfactory as the 31 to 1 proportion. Likewise a 35 to 1 proportion will still produce a coating, but likewise not entirely satisfactory.

If the solution as'a whole is diluted, the proportion of total acidityto free acid is increased. As the activity of the solution is closely related to the free acid, a dilution of the solution from the proportions given slows it up noticeably. On the other hand an increase in the strength of the solution, maintaining the porportions the same, results in hydrolyzation giving a greater amount of free-acid in proportion to total acidity. For these reasons, while the solution may be either stronger or weaker than that given and still produce coatings, the strength specified is found preferable for the combination of speed and economy, where the solution is to be used as a bath. When the article is removed from the bath, some of the solution clings thereto. The richer this solution is, the more chemicals are removed in this manner, and it is not easy to economically recover these chemicals. A point may be reached in the concentration of the solution where a syrupy condition increases the difiiculties. For these reasons it is not advisable to increase the strength of the solution more than is needed for the desired speed of reaction.

The proportion of copper nitrate to zinc phosphate may be varied somewhat. With the proportions given, the solution will contain about 18/100ths of 1% of copper. If the proportion of copper nitrate is increased so that, with the solution otherwise the same, there is over 2 tenths.

of 1% of copper in solution, the coating is not as satisfactory, ,being less firm and adherent. On the other hand, if the copper is reduced below 12/100ths of 1% with the solution otherwise the same, the results are not satisfactory. Of course the amount of copper can be increased or decreased beyond the limits given if the phosphate is similarly increased or decreased, in accordance with the statements above concerning the dilution or enriching of the solution.

In the solution given, about 16/100ths of 1% of copper, or a little more, is the preferred amount, but the copper works out of the solution faster than the zinc phosphate, so that starting with about 18/100ths of 1% of copper in solution, it will be reduced by the use of the solution, and may be reduced several hundredths of 1% without noticeably decreasing the efficiency of the solution. Because of this faster working out of the copper, when replacement is necessary a higher percentage of copper is employed in proportion to the zinc phosphate than in making up the original solution. It has been found best to replenish frequently because the proper operation of the solution is obtained only within comparatively narrow limits. For this reason it is deemed advisable to replenish as soon as the solution drops as much as one point in total acidity from the ordinary amount. It requires about 2 grams of zinc phosphate to about 1 /2 grams of cupric nitrate to 4 liters of water to raise the total acidity of the solution one point, and the material used in about that proportion in replenishment maintains substantially the same proportions of ingredients in the bath, as cupric nitrate andzinc phosphate are apparently ex hausted inabout the ratio of 3 to 5, with the solution maintained in the proportions specified.

In the above solution, zinc phosphate is mentioned, and this phosphate is considered preferable to manganese phosphate or iron phosphate both in starting and replenishing the bath. It has been found that. zinc phosphate does not hydrolyze so as to produce as much free acid in proportion to total acidity at lower temperature as does manganese phosphate or iron phosphate. On the other hand the zinc phosphate hydrolyzes somewhat more and produces a somewhat higher percentage of free acid to total acidity than does manganese phosphate or iron phosphate when 8( the temperature is raised to boiling. However, in a boiling solution zinc phosphate seems to be somewhat more active than the manganese phosphate or iron phosphate, and will give a satisfactory coating with a higher percentage of free acid than will a solution formed from manganese phosphate or iron phosphate. As a result is has been found that the hydrolyzation of zinc phosphate in the solution specified gives substantially the most satisfactory proportion of free acid to total acidity at any temperature at which it is desired to employ the solution. If manganese phosphate is employed in place of zinc phosphate, the reaction at lower temperature is not .as prompt and the free acidity is too high unless 96 some material is introduced to neutralize some of the free acid. Such neutralization of the free acid requires some technical skill and is also wasteful of the materials. For this reason, while a solution formed from manganese phos- 100 phate may be used if zinc phosphate is not avail-' able, the manganese phosphate is not as emcient nor as easily kept within control.

drolyzation is kept within working limits.

As previously noted, the hydrolyzation ,of the phosphates increases with increased temperature. If the temperature is maintained constant at the elevated point, the reaction taking place at that temperature will produce a satisfactory coating with an increase of free acidity. However, if the solution is cooled off after being heated, it will contain a higher free acid contact than is required at that reduced temperature, or than is produced when the solution is maintained at the reduced temperature. For this reason, a solution that has been heated up and then cooled down does not give satisfactory results until the. free acidity has been neutralized to come within the working range at the cooler temperature or un-' til it is reheated to raise the temperature to that appropriate for its proportion of free acid. The

variation in the percentage of acid permissable in the solution is sufficient relative to the change resulting from different temperatures so that no difficulty is experienced in changes due to ordinary variation in room temperatures. The above 3 warning concerning the use of a cooled solution would be applicable particularly if the solution was heated up to bring quick results for a time, and then allowed to cool 01! and used at low temperature, in which case partial neutralization might be necessary to reach the proper proportions of acidityat the cooled temperature.

Other' soluble compounds of copper might be utilized, and oxidizing agents other than nitric acid, or a nitrate other than cupric nitrate, might trate and cupric acetate. When introduced in chemically equivalent amounts, these materials will work satisfactorily, as the sodium of the nitrate and the acetic acid of the cupric acetate are not initially injurious to the bath.

While copper has been found to be very satisfactory asa metal less basic than those to be coated and to act as an accelerator for the coating operation, other metals less basic than the metal to be coated may be employed by proper .variations in the proportions of the materials and acidity, etc. However, such metals as mercury are plated onto the metal to be coated so rapidly that they are difficult to control, and, while lead may be employed in combination with copper, it is difficult to obtain a solution in which the coating is as firm and adherent as where copper is employed. The high cost of silver renders its use inadvisable. For that reason preference is given to copper as the metal less basic than the metal to be coated.

A coating produced by the method and solution described above is more resistant to rusting than a coating produced in a bath containing copper without an oxidizing agent.

This phosphate coating is advantageous because it is substantially integral with the article and it forms a much better bond with paint or enamel than does clean smooth metal. As a result of this better bond, any pin holes through the coating of paint or enamel, or any scratches or accidentally formed regions where the enamel or paint is removed, are surrounded by the paint or enamel firmly adherent to the phosphate coating which in turn is substantially integral with the metal of the article and, therefore, there is no opportunity for rust to spread between the paint or enamel and the metal of the article, as frequently happens in the absence of such a bond between the paint or enamel and metal.

The solutions described herein operate even more promptly upon zinc surfaces than upon iron surfaces, and likewise may be used on surfaces of magnesium or its alloys.

While the general principles upon which the modification of the solution may be made while still carrying out essentially the same process and obtaining a satisfactory coating have been outlined, there are other details of modification, by the use of equivalent chemicals which will occur to those skilled in the art, which may be made within the scope of the appended claims.

What we claim is:

1. The method of coating metal which consists in forming a bath containing phosphate of metal of the group comprising zinc, manganese and iron, a soluble compound containing a metal less basic than the metal to be coated, and an oxidizing agent, maintaining the bath at room temperature, and immersing in the bath an article having a surface composed of iron, zinc, magnesium or alloys thereof.

2. The method of coating metal which comprises forming a bath containing di-hydrogen phosphate of metal of the group comprising zinc, manganese and iron, a soluble compound containing a metal less basic than the metal to be coated, and an oxidizing agent, and having its proportion of acidity as indicated by a phenolphthalein indicator to its acidity as indicated by methyl orange indicator between 25 to 1 and 35 to 1, maintaining the bath at room temperature, immersing therein articles having a surface of iron, zinc or magnesium or alloys thereof, and replenishing the bath to maintain a substantially constant proportion of the ingredients thereof.

3. The method of coating metal which consists in forming a bath comprising an aqueous solution of zinc and ferrous di-hydrogen phosphates and cupric nitrate and containing about five times as much phosphate as nitrate, immersing in the bath articles having surfaces of iron, zinc or magnesium or alloys thereof, and replenishing the bath by adding zinc phosphate and cupric nitrate in proportion of 5 to 3.

4. A solution for coating metal of the group comprising iron, zinc and magnesium, comprising an aqueous solution of an acid phosphate of metal of the group comprising zinc, manganese and iron, a soluble compound of a metal less basic than the metal to be coated, and a nitrate, the total acidity as measured by phenolphthalein indicator being at least fifteen times the free acidity as measured by methyl orange indicator.

5. A solution for coating metal comprising an aqueous solution of zinc di-hydrogen phosphate and a soluble compound of copper and an oxidizing agent, the proportions of the ingredients giving, by normal hydrolysis at F., a proportion of acidity as indicated by phenolphthalein indicator to acidity as indicated by methyl orange. between 25 to 1 and 35 to 1.

6. A solution for coating metal comprising an aqueous solution of zinc di-hydrogen phosphate and cupric nitrate.

7. A solution in accordance with claim 6 in which the proportion of phosphate to nitrate is approximately 5 to 1.

ROBERT R. TAN'NER. HERMAN JOHAN LODEESEN. 

